Networks for mobile telecommunications apparatuses are known.
These networks are generally described as cellular and they are distinguished by a plurality of cells, each defined as the group of territorial points served by the radio-electric signal radiated by an antenna (radio interface).
Apart from the intrinsic mobility of users, the main peculiarity of networks of mobile apparatuses is the use of the radio interface as access port to the network itself.
It is also known that dimensioning and performance assessments of telecommunications networks or systems in which the offered traffic is composed of voice and data traffics, can be evaluated by using the method described in WO 02/080602. The known method is based on an M/M/N/Q model in which the system servers correspond to the cell traffic slots and the status of the cell is represented by the number of GPRS users in the system; this model takes into account:                a queue of infinite length (Q=∞) on the basis that, in the case of congestion of the traffic resources, each GPRS user makes multiple access attempts to the system;        an average queue waiting time (for one GPRS user) expressed as a function of the time necessary for the mobile terminal to make the multiple access attempts to the system;        that the entire message is deemed to have been put in the queue;        an inter-arrival time between the data calls (arrivals process) having exponential distribution with parameter λ equal to the frequency of the arrivals;        an average call duration (service time τ) having exponential distribution with parameter μ=1/τ equal to the call death intensity;        an average waiting time in the queue having exponential distribution with parameter a equal to the frequency of the dropped calls (user “impatience”).        
As far as the characteristics of the GPRS data service are concerned, the known method takes into account that:                each user (or, rather, each GPRS mobile terminal) attempts to access the system, in accordance with various policies, for a predetermined number of seconds with multiple attempts; if the user has been unable to access the radio slot at the end of this time period, the call is blocked;        access to the radio slot in the system is gained, in a known manner, on a call basis and not for the individual message packet;        the transmission speed of the message, after obtaining the radio resource, depends on the number of users multiplexed on the GSM time slot; this number goes, for example, from a minimum of one user to a maximum of eight; as a consequence of this, the speed, as is known, can vary during transmission of the single message on the basis of the number of users who access the GSM-GPRS cell.        
According to the known method the data traffic “A” offered to the cell is evaluated by means of the following relation:
  A  =            λ      μ        =          λ      ·      τ      where:
  τ  =            nL              v        canal              =          1      μ      minimum service time of a data call;and    vcanal speed of the single server;    n·L length of the message (n packets of length L).
By knowing the number of slots available at any moment for the GPRS service, the set of possible status associated with a cell is then summed up in flow balancing equations required for calculating the various status probabilities, given by the relation:
      P    x    =            P      0        ⁢                            λ          0                ⁢                  λ          1                ⁢                                  ⁢        …        ⁢                                  ⁢                  λ                      x            -            1                                                μ          1                ⁢                  μ          2                ⁢                                  ⁢        …        ⁢                                  ⁢                  μ          x                    where P0 is the probability of the system being in status 0; the set of probabilities is then normalised by means of the P0 normalisation relation, corresponding to the formula:
      P    0    =                    (                  1          +                                    ∑                              k                =                1                            ∞                        ⁢                                          ∏                                  i                  =                  0                                                  k                  -                  1                                            ⁢                                                          ⁢                                                λ                  i                                                  μ                                      i                    +                    1                                                                                      )                    -        1              .  
In the known system, the voice traffic has priority over the GPRS data calls (preemption) and it is necessary to weigh up the various possible configurations of slots available for the data service with the probability, linked to voice traffic only, of each configuration effectively occurring. For this purpose, the state of the art makes it possible to assess the probabilities of having x channels or slots left free by voice, and therefore usable by the GPRS service, through the relation:
  {                                                        P              D                        ⁡                          (              x              )                                =                                    P              V                        ⁡                          (                              C                -                x                            )                                                            1          ≤          x          <          D                                                                        P              D                        ⁡                          (              D              )                                =                                    ∑                              i                =                0                                            C                -                D                                      ⁢                                          P                V                            ⁡                              (                i                )                                                                                                 where C represents the number of channels of a cell, D corresponds to the maximum number of channels allocable for data (static plus dynamic) and Pv(i) is the probability of having “i” channels occupied by voice, given by the relation of a known kind:
            P      V        ⁡          (      i      )        =                                          (                          A              voice                        )                    i                          i          !                                      ∑                      j            =            0                                C            -            1                          ⁢                                            (                              A                voice                            )                        j                                j            !                                .  where Avoice represents the voice traffic offered to the cell.
The effective performance of the cell (probability of data block and user throughput) is therefore given by the following relations:
      B    D    =            ∑              x        =        1            D        ⁢                  B        ⁡                  (          x          )                    ·                        P          D                ⁡                  (          x          )                    average probability of data block;
      R    D    =            ∑              x        =        1            D        ⁢                  ET        ⁡                  (          x          )                    ·                        P          D                ⁡                  (          x          )                    average data delay;where B(x) and ET(x) are the average data loss probability and the average delay associated to the configuration with “x” channels available for data traffic.
The user throughput is calculated on the basis of the average delay, by means of the relation (of known type):
      n    ·    L        R    D  with message length n·L.
In the specific case of the GSM-GPRS network, data traffic for the various types of service is managed (served) using radio carriers of predefined frequency and, in the framework of each radio carrier (FDMA access technique), by a given slot (the logic channel) among those periodically available in the framework of the time frame used on the radio interface (TDMA access technique). In this context, if even one user requests a data transmission, one or more whole slots of the GSM time frame are assigned to the user as a function of the terminal capability (preferred number of time slots requested to the network) of the mobile station; this implies a given transmission speed, for example 9.05 kbit/s nominal per slot, for the data encoding denominated CS-1, or 13.4 kbit/s nominal per slot for the date encoding denominated CS-2.
If, on the other hand, several users simultaneously request data transmission, one or more slots are subdivided among the users themselves, with a consequent drop in the transmission speed which will therefore be a function of the number of active users in the system (cell) at that moment; in a more intuitive manner, the generic user notes a net data transmission speed which varies with time on the basis of the load conditions in the system.
In other words, the known method for determining the call death probability associated to a determined network status is based on considering that each terminal on the cell uses a same capability.
From Applicant's analysis of the known method, it emerges that the methodology for assessing dimensioning and performance of the base stations of a network for mobile telecommunications apparatuses is inadequate.
In fact, a cell of a real network comprises data terminals requesting different numbers of time slots to the network for transferring data.